The present invention relates to a novel system and novel method for controlling and optimizing irrigation and leaching of agricultural plants.
In commercial nurseries and in other commercial venues where agricultural plants are raised, growers seek to control irrigation of their plants. Motivation for practicing efficient water use includes optimizing water use for irrigation and controlling mineral leaching from soils or soilless substrates during irrigation. Agricultural plants, for example, nursery plants, turf grasses, and commercially grown plants, may be stressed by over watering or under watering. For any particular environmental situation, there is an optimal amount of water required. The optimal amount of water depends on a number of factors, including the time of the day at which the irrigation is done, the frequency of irrigation, the specific crop being irrigated, time of the year, climate, soil and substrate physical properties, and container capacity. A number of methods have been utilized for irrigation control, including electronically or mechanically controlling a water source based on flow rates of the water, weight of nursery pots, and monitoring environmental factors such as temperature and rainfall. Typically, a grower seeks to determine the optimum amount of water to apply to an agricultural plant by either placing a sensor in the soil to determine when the soil has received enough, but not too much water, or by measuring the amount of excess water exiting a contained plant.
Freeman, et al (U.S. Pat. No. 7,123,993), teaches remote control of an irrigation system including a weather sensor to detect whether there has been rain, designed so the operator may control the system through the internet.
Moore, et al (US Publication No. US 20060161309), teaches an irrigation control system for controlling irrigation based on weather data that can receive input data from various sensors.
Stadelhofer (U.S. Pat. No. 3,786,598), teaches the use of a method of using a wick placed in the soil of potted nursery plants connected to a water source beneath the potted nursery plants. The plant draws sufficient water via the wick to keep it properly watered.
Irrigation timing can also be determined by monitoring containers gravimetrically and irrigating based on percentage of container capacity. (See Kaprielian and Adding Infra.) Container capacity is defined as the proportion of substrate volume filled with water after saturation followed by drainage, or, in other words, the maximum amount of water a substrate-filled container can hold against gravity (Fonteno, W. C.; “Growing Media: Types and Physical/Chemical Properties”; In: A Grower's Guide to Water, Media, and Nutrition for Greenhouse Crops (David William Reed, Editor), 1996; Page 93-123; Ball Publishing; Batavia, Ill., US). However, it is not clear what the optimum percentage of container capacity is for plant growth and health. Further, a method based on weight of the container must be recalibrated periodically to account for changes in the base mass of a plant as the plant grows.
Addink (U.S. Pat. No. 6,102,061), teaches using an irrigation controller with pre-set instructions for watering, which is an example of a timed irrigation system.
Evapotranspiration (ET) modeling also has been used for determining irrigation timing. This approach allows the actual ET of a crop to be estimated by measuring ET based on environmental parameters and adjusting it with a specific crop coefficient (Beeson, Richard C.; “Modeling Irrigation Requirements for Landscape Ornamentals”; HortTechnology; January-March 2005; Volume 15; No. 1; Page 18-22; American Society for Horticultural Science; Alexandria, Va., US). This approach requires correlating the ET of a specific crop with the ET of a reference crop, and then correlating these ETs to a crop-specific parameter, such as time since transplant. However, because the relationships often vary considerably even within a species, finding reliable crop coefficients for container-grown woody ornamentals has proven difficult and extremely laborious, and thus ET-modeling has been impractical for most nursery operations.
One example of an application using evapotranspiration is shown in Kaprielian (U.S. Pat. No. 7,937,187). Kaprielian teaches a system and method of computer controlled irrigation and fertigation. As part of its teachings, Kaprielian discloses that his method relies on determining water consumption by a plant by measuring the difference in weight of the water added to a container and the weight of the water exiting the container, or the excess water. These measurements are analyzed by a central processor and from that analysis a predetermined amount of water is used. Kaprielian also teaches irrigation control based on certain chemical parameters.
Another method for controlling irrigation comprises monitoring of substrate, for example soil moisture content using tensiometers or electric probes. Thaxton compared growth, effluent volume, and nutrient uptake for trees irrigated using a switching tensiometer control system compared to cyclically timed treatments, wherein a tensiometer sent a signal to open and close irrigation valves when predetermined moisture levels were attained (Thaxton, V. V.; “Irrigation Management in Large Container-Grown Woody Ornamentals”; M.S. Thesis; 2001; Louisiana State University; Baton Rouge, La., US). Similarly, Nemali, et al used dielectric moisture probes embedded in container substrate to monitor substrate moisture content (Nemali, Krishna S., et al; “An Automated System for Controlling Drought Stress and Irrigation in Potted Plants”; Scientia Horticulturae; 2006; Page 292-297; Volume 110; Elsevier B.V.; Amsterdam, Netherlands). Sensor output was used to control irrigation to maintain a specific moisture level. Murray, et al used a time-domain-reflectometry system to provide measurement of substrate moisture content (Murray, J. D., et al; “Time Domain Reflectometry Accurately Monitors and Controls Irrigation Water Applications in Soilless Substrates; ISHS Acta Horticulturae; XXVI International Horticultural Congress: Protected Cultivation 2002: In Search of Structures, Systems and Plant Materials for Sustainable Greenhouse Production; Volume 633; Page 75-82; International Society of Horticulture Science; Korbeek-Lo, Belgium). However, because substrate moisture may not be uniform throughout the container, results may vary and thus are problematic for commercial application.
Irrigation may also be controlled by monitoring an effluent or leachate exiting a container. In such a system, a precipitation gauge or moisture sensor is located beneath a container to detect an effluent, and the signal is used to control irrigation. In one study, a computer-controlled drip irrigation system was based on presence of effluent by suspending containers above a moisture sensor connected to a datalogger (González, Rico A., et al; “A Computer-Controlled Drip Irrigation System for Container Plant Production”; HortTechnology; July-September 1992; Volume 2; No. 3 Page 402-407; American Society for Horticultural Science; Alexandria, Va., US). The datalogger controlled irrigation valves so that upon commencement of leaching, solenoids could immediately be closed. In a system such as this, total effluent was collected and measured to determine application efficiency. Precipitation gauges measured effluent volume during leaching, which may allow for predictive control of specific leach volumes, adding precision to a practice that has historically been rather ambiguous.
However, none of the methods disclosed provides a quick, inexpensive, and reliable system or method for optimizing irrigation.
Leaching
In addition to reducing wasted water, excess water may damage container plants by leaching necessary nutrients. If plants are over watered, excess water flowing through the planting substrate may deplete soil, or soilless substrates, through leaching of necessary nutrients, for example, minerals such as nitrate salts, ammonium salts, and phosphorous-containing compounds. Excess irrigation also may cause, for example, unwanted changes in pH or electrolytic conductivity of leached soil. Thus, failure to control irrigation may cause uncontrolled leaching of soil, which may be detrimental to plant growth.
However, if excess fertilizer is applied to plants, leaching may be desired. For example, to reduce labor costs, growers may apply controlled release fertilizers (CRFs) less often, but in larger quantities. If CRFs break down at rates quicker than plant uptake, detrimental salt concentrations in the container can result. This problem may be exacerbated if, to save water, one uses one of the methods described above. In those cases in which substrate salinity rises to toxic levels, leaching may be used to return the substrate to acceptable conditions.
The amount of water needed to flush excess salts from the substrate depends on a number of factors, including salinity of irrigation water, percolation rate of irrigation water through a substrate, hydraulic conductivity of a substrate, and amount of water exiting a substrate. The salinity of irrigation water affects the ability of substrate solutes to diffuse from high concentrations in the container to low concentrations in the irrigation water. The percolation rate affects how much time the irrigation water spends mixing with the substrate solution, and depends on the application rate as well as the physical properties of the substrate. Longer rates lead to more intimate mixing, and thus more removal of salts from the profile. Substrate conductivity affects how thoroughly the soil profile is wetted, and is affected by substrate physical properties as well as moisture content. Only substrate that is wetted can be leached. Better conductivity will allow more lateral movement of water and more thorough wetting of substrate. Lastly, higher volumes of water passing through the substrate can carry away larger amounts of solutes. Because of the several variables associated with leaching, it appears that the best measure for leaching effectiveness was to base leaching effectiveness on container capacity rather than the volume of irrigation water.
One approach to control leaching has been to determine a Leach Fraction (LF), where Leach Fraction=Volume of Effluent÷Volume Applied Water. While this approach is simple and relatively inexpensive, it may not always be effective in preventing over or under irrigation. Although it is known that total nutrient loads lost are lower with reduced leach fractions, there is no consensus on what the proper leaching fraction for container crops should be. There are a number of variables associated this approach, many of which are difficult to monitor or control. For example, there may be significant time delays between the time that irrigation is turned off and the time that leaching actually stops, substrate composition may differ, antecedent substrate moisture may not be known, different container size may affect LF, and plant rooting may have a significant effect on LF. LF normally is maintained between 0.10 and 0.60, although this simple approach is not reliable in all cases for controlling leaching. LF typically has been restricted to small container crops, and has not been used for commercial larger, canyard-size crops.
Use of LF has been most successful when growers employ gravimetry or evapotranspiration methods to determine actual daily water use and actual daily water loss. However, as described above, existing methods for measuring water use and loss are complex, expensive, and lack certainty. Thus LF methods currently in use do not appear to be adequate for practical use for most growers. A simpler, less expensive and more consistent method is needed in order for growers to implement good leaching practices in the commercial industry.